1. Field of the Invention
The present invention relates to a coke drum mounting and support skirt, and more particularly to a novel support skirt that allows for expansion and contraction of the coke drum during the extreme temperature changes experienced by the coke drum during the delayed coking processes. The described support skirt securely supports the coke drum and prevents tipping of the drum, while allowing thermal contraction and expansion without undue stress to the support or drum.
2. Background and Related Art
Many oil refineries recover valuable products from the heavy residual hydrocarbons (commonly referred to as resid or residuum) that remain following initial refining by a thermal cracking process known as delayed coking. The processing of crude oil into gasoline, diesel fuel, lubricants, and the like, as well as many other petroleum-refining operations, produces byproducts that have very little value. However, the value of these byproducts can be substantially increased when they are heated for a long enough time at a temperature sufficient to cause “destructive distillation.” During the process of destructive distillation, a portion of the byproducts is converted to usable hydrocarbon products. The remainder is transformed into a solid carbon product called coke. In the refining industry, this process is commonly known as delayed coking.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke vessel where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke vessel, which typically consist of steam, gas, naphtha and gas oils, are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Normal operating pressures in coke vessels typically range from 25 to about 50 pounds per square inch and the heavy feed input temperature may vary between 800 degrees Fahrenheit and 1000 degrees Fahrenheit.
Coke drums are typically large, cylindrical vessels commonly 19 to 30 feet in diameter and up to 120 feet tall having a top head and a funnel shaped bottom portion fitted with a bottom head and are usually present in pairs so that they can be operated alternately. The size, shape, and configuration of the coke drum may vary considerably from one installation to another. Coke is formed and accumulates in the vessel until it is filled to a safe margin, at which time the heated feed is switched to the empty “sister” coke vessel. This use of multiple coke drums enables the refinery to operate the fired heater and fractionation tower continuously. Thus, while one coke vessel is being filled with heated residual oil, the other vessel is being cooled and purged of coke (between 500 and 1200 tons) formed in the vessel during the previous recovery cycle. The full vessel is isolated, steamed to remove hydrocarbon vapors, cooled by filling with water, drained, opened, and the coke is removed. The drums typically operate on a cycle, switching every 10 to 30 hours.
Coke removal, also known as decoking, begins with a quench step in which steam and then water are introduced into the coke-filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke. The vessel is then drained and vented to atmospheric pressure then opened (unheaded or deheaded) in preparation for decoking. Decoking is accomplished at most plants using a hydraulic system consisting of a drill stem and drill bit that direct high pressure water jets into the coke bed. This cuts the coke into small pieces which fall out the opened bottom of the coke drum. Once it is decoked, the drum is closed (re-headed), purged of air, leak tested, warmed-up, and placed on stand-by, ready to repeat the 10- to 30-hour cycle.
The coke drums are largely vertical, with heights from three to four times their diameters to facilitate the delayed coking process and the decoking process. This large height/diameter ratio makes the coking drums susceptible to tipping due to forces such as those from strong winds. Further compounding this problem, the coke drums must be elevated to some extent to allow room underneath the coke drums for the dislodged coke to fall out and be removed during the decoking process. This increases the susceptibility of the coke drums to winds and other forces.
The coke drums must be secured against these forces. A typical coke drum is supported by a skirt which is welded to the drum near the junction of the drum shell and the lower cone of the drum. The skirt of the coke drum is then typically placed on a reinforced cylindrical or quasi-cylindrical hollow concrete base that provides support for the drum. This is necessary due to the extreme weight of a filled steel coke drum containing as much as 1200 tons of coke and built to withstand over 50 pounds per square inch of pressure at 900 degrees Fahrenheit. The coke drum's skirt is typically bolted to the concrete base with heavy bolts along the base of the skirt.
This is problematic, however, for the cyclical coking/decoking process subjects the large and heavy coke drums to frequent temperature fluctuations of hundreds of degrees. The temperatures fluctuate from the decoking temperature which may approach environmental conditions of 100 to 200 degrees Fahrenheit to the operating temperature around or above 900 degrees Fahrenheit. The steel drums, of course, expand and contract as a result of the temperature changes, and this expansion and contraction can be quite severe. For example, an unsecured thirty-foot-diameter steel coke drum may increase in diameter as much as two to two and one-half inches during the 700-800-degree-Fahrenheit temperature change it experiences during delayed coking and decoking.
The typical coke drum, however, is not unsecured, but is securely bolted at its base to prevent tipping. The typical bolting process severely restricts the range of expansion within which the base of the coke drum can expand. This fixed securing structure results in large forces and stresses at the base of the coke drum. The bolts securing the skirt to the concrete base may be subjected to large shear stresses as the coke drum attempts to expand, which may eventually result in failure of the bolts. In addition, the joining of the skirt to the coke drum also undergoes large stresses and is subject to failure, which may lead to rupture of the shell of the coke drum. In addition, the concrete in which the bolts are embedded may crack and fail due to the stresses incurred. Finally, another potential hazard exists. The failure of the system securing the coke drum to the concrete base may be slow and almost invisible, resulting in a gradual weakening of the support system. While the support system might appear to be fine externally, the weakened support system may no longer be able to support the drum in high winds or other lateral forces, leading to sudden, unexpected, and catastrophic failure.